Stereopsis may be described as the ability to appreciate depth binocularly and to recognise the specific localization of form and its relative position to the observer and to other spatial objects. In layman's terms this amounts to three dimensional vision. This facility is not shared equally by all people. The inability to appreciate depth of vision binocularly may be caused by conditions such as strabismus or for other less apparent reasons. Approximately 95% of people enjoy some measure of stereopsis.
Stereopsis is regarded as one of the highest functions of the vision system and seems to have developed phylogenetically at the same time as the emancipation of man's hands. It is the coordinated visual control of man's hands which has been largely responsible for his intellectual and technological progress. It is well known that the visual acuity of sighted people varies and bears some relationship to their refractive states. In the same manner it has been found that people present with different degrees of stereo acuity, that is different degrees to which objects may be perceived to lie in different planes when viewed binocularly.
The retinal system is known to operate with many sub-systems which are receptive to movement, colour, edges, light intensities and contours which contribute to depth perception. Stereopsis does not rely on vision perception alone but calls for an integration of all of man's senses and motor systems to answer the questions of what, where and when objects are present in space. Stereopsis is accordingly regarded as the highest form of binocular vision and stereoscopic localization appears to be one of the first discriminations of which the human visual system is capable.
It is further known that there are many monocular cues which contribute to the appreciation of depth. However, since the late 1950's it has also been known that stereopsis is indeed possible in the absence of monocular form cues.
The increasing knowledge and understanding of the anatomy and physiology of the visual pathways provides researchers with a continually increasing understanding of the manner in which binocular vision is attained through the integration by binocular cells in the visual cortex of images that fall on corresponding regions of the retinae of both eyes. The role of the optic chiasma in this respect is now well understood. It is also known that the corpus callosum plays a role in stereopsis and that through these connections, disparate image information in each hemisphere of the brain is brought together to see stereoscopically. If the optic chiasma is cut longitudinally along its midline, this will give rise to bitemporal hemianopia and also a blind area beyond the point of regard. However, the patient will still be able to appreciate stereopsis in front of the plane of regard. If the corpus callosum is cut along its midline, as is done in some cases of Parkinson's disease or epilepsy, then this gives rise to local stereo-blindness within a narrow angle in front of, as well as behind, the plane of regard.
Vision care practitioners have for the past century used devices to determine whether their patients are able to see stereoscopically. One such device consists of the use of polarized stereo-vectograms which are used in conjunction with dissociating polarized viewers. Besides providing information about the quality of a patient's binocular vision, these tests may also be used to assist in the diagnosis of strabismus. Stereograms produced for this purpose are sold commercially by several manufacturers including Stereo Optical Company, Inc. of Chicago, Ill., United States of America. The most commonly used stereograms fall into two groups, namely those featuring contours which provide monocular cues as to the form or symbol in the stereograms, and those without such cues, which latter group is known as random dot stereograms. When viewing a stereogram of this nature through dissociating polarised lenses the patient with normal stereopsis perceives images of objects in the stereogram to be displaced either forwardly or rearwardly of the plane of regard or fixation being looked at. This illusion is achieved by creating on the stereogram a form having stereo disparity. The larger the degree of stereo disparity of the symbol being observed, the further it appears to be displaced relative to the plane of regard and hence the easier will it be for the patient to discriminate between the form and its reference ground. The ability to see the apparent displacement can hence be used as a measure of the patient's stereo acuity.
It is believed that valuable information about a patient's visual functions and ability to respond to visual stimuli can be obtained by means of an assessment of his or her stereo acuity both in static and dynamic mode. Such information can then be used to assess a person's spatial abilities which appear related to the level of his stereo-acuity and the speed with which he or she makes such judgments. The information may also be used to assess the value of any therapeutic regimen and its effects on the speed and acuity of stereopsis, e.g. medication, exercises or lenses. To date the available tests have not been standardized to allow for cross comparisons and to assess different stereoscopic performance conditions accurately.